Negative overvoltage protection circuit, in particular for output stages

An output stage with a protection circuit against negative overvoltage at its output terminal, comprising:

• a negative overvoltage protection circuit including a first protection diode, said first diode having a first terminal connected to said output terminal of said stage and a second terminal;
• an output transistor having a collector connected to said second terminal of said first protection diode, a base and an emitter;
• control means connected to said base to actuate said output transistor between a substantially conducting state and a substantially nonconducting state;
• means interposed between a positive supply voltage line and said output terminal to bias said negative overvoltage protection circuit and said output transistor;
• and
• a voltage generating circuit, connected between said emitter and a group reference line, comprising voltage source means, for generating a reference voltage the value whereof can be preset with respect to a ground reference line to substantially compensate for a voltage drop across said negative overvoltage protection circuit, said voltage generating circuit including:
• an operational amplifier in a voltage-follower configuration, said operational amplifier having an output connected to said emitter of said output transistor;
• an inverting input connected to its output;
• and a non-inverting input;
• a second diode connected to said ground reference line;
• said voltage source means with a preset value which is connected between said second diode and said non-inverting input of said operational amplifier;
• and
• a current source which is connected between said non-inverting input of said operational amplifier and a negative supply line of said stage.

An output stage with a protection circuit against negative overvoltage at its output terminal, comprising:

• a negative overvoltage protection circuit including a first protection diode, said first diode having a first terminal connected to said output terminal of said stage and a second terminal;
• an output transistor having a collector connected to said second terminal of said first protection diode, a base and an emitter;
• control means connected to said base to actuate said output transistor between a substantially conducting state and a substantially nonconducting state;
• means interposed between a positive supply voltage line and said output terminal to bias said negative overvoltage protection circuit and said output transistor;
• and
• a voltage generating circuit, connected between said emitter and a ground reference line, comprising voltage source means, for generating a reference voltage the value whereof can be preset with respect to a ground reference line to substantially compensate for a voltage drop across said negative overvoltage protection circuit, said voltage generating circuit including:
• an operational amplifier in a voltage-follower configuration, said operational amplifier having an output connected to said emitter of said output transistor;
• an inverting input connected to its output;
• and a non-inverting input;
• a second diode connected to said ground reference line;
• said voltage source means with a preset value which is connected between said second diode and said non-inverting input of said operational amplifier;
• and
• a current source which is connected between said non-inverting input of said operational amplifier and a negative supply line of said stage,
• wherein said first protection diode and said second diode have substantially the same voltage drop across their terminals when active.